1. Field of the Invention
The present invention relates to a sheet-material transporting device for transporting sheet material while allowing the sheet material to be attracted to a transporting belt, and further relates to an image forming apparatus including the sheet-material transporting device.
2. Description of the Related Art
Image forming apparatuses, such as printers, copiers, and facsimiles, form an image composed of a pattern of dots on sheet material (i.e., a recording medium such as paper or a thin plastic sheet) on the basis of image information. Such image forming apparatuses can be divided, according to their recording method, into several types, including an inkjet type, a wire dot type, a thermal type, and a laser beam type. Of these types of image forming apparatuses, an inkjet-type image forming apparatus is configured to eject ink from a recording head onto sheet material, such as recording paper, thereby forming an image on the sheet material. The inkjet-type image forming apparatus facilitates the compactness of the recording head, and can provide high-definition images at high speed and low running cost. Moreover, the inkjet-type image forming apparatus, which is a non-impact type apparatus, produces less noise and can easily record color images with multicolor ink. In particular, with a full-line type recording apparatus including a line-type recording head having many orifices arranged across the paper width, it is possible to further increase the speed of recording.
Generally, an image forming apparatus transports sheet material from a paper feed unit (e.g., a paper cassette), through an image forming unit (recording unit), to a paper ejecting unit. The transport of the sheet material is controlled at predetermined timing throughout the process from paper feeding, through image formation, to paper ejection. The process from paper feeding to image formation particularly requires accurate transport, as an image forming position on the sheet material will be affected. Moreover, during image formation, if the sheet material is not transported at a constant speed, a deviation in image scaling factor occurs and causes undesirable expansion or contraction of an image. In particular, for an image forming apparatus with a plurality of recording heads, a displacement between images recorded by the plurality of different recording heads occurs. In the case of a color image forming apparatus, this displacement appears as a color displacement and causes image defects. To prevent such problems, it is necessary to accurately transmit the transporting force of a precisely controlled transporting unit to the sheet material.
An example of transporting systems proposed in view of the above-described aspects is a transporting device that includes an endless belt and using an electrostatic attraction force to bring sheet materials into close contact with the endless belt. For such a belt-type transporting device using an electrostatic attraction force, and particularly for a color image forming apparatus with a plurality of recording heads (image forming units), it is necessary to precisely maintain the transporting speed of the belt for accurate adjustment of an image forming position for each recording head. It is also necessary to hold sheet material in close contact with a transporting member (such as a belt or a drum) so that the sheet material on the transporting member can be prevented from floating or being displaced.
However, in an image forming apparatus, such as a color image forming apparatus with a plurality of long full-line type recording heads extending in a direction intersecting the transporting direction, a distance from a recording head on the uppermost stream side to that on the lowermost stream side is very long. This causes flapping of sheet material in a recording area and may result in distortion in recorded images and paper jams. A method proposed for urging sheet material downward to prevent it from floating is to apply a voltage to electrodes included in a transporting belt to generate an electric force, thereby causing the sheet material to be attracted to the transporting belt. Other proposed methods include a method in which an electrostatic attraction force is used to cause sheet material to be attracted to the transporting belt, and a method in which a pressure control chamber is provided to regulate pressure by a fan, thereby attracting sheet material to the transporting belt.
Japanese Patent Laid-Open No. 2000-247476 and Japanese Patent Laid-Open No. 2000-60168 discuss the above-described methods in which sheet material is transported while being attracted to the transporting belt in the recording area, and is subjected to recording performed by the recording head.
However, the above-described techniques present technical challenges to be solved. For example, in a method in which an electrostatic attraction force is used, it can be difficult to generate an attraction force sufficient for reducing cockling. Moreover, a variation in electrical characteristics caused by image formation can make it difficult to maintain a stable attraction force. In a transporting device using the other attracting method described above, since sheet material is attracted solely at an opening, and not attracted by an attraction-force generating unit provided over an extensive area, it can be difficult to attract an end portion of the sheet material. Moreover, it may be possible that the attraction of air causes image degradation, because air passing through sheet material may contain ink mist and cause ink spots on the sheet material.
On the other hand, a transporting device using an electrostatic attraction force described above is configured such that the uppermost stream end of a charging brush is located at substantially the same position, in the transporting direction, as that of the nip of a pressing roller for pressing sheet material against a transporting belt. As a result, there may be cases where sheet material transported from correction rollers for aligning the edges of the sheet material and compensating for the skew thereof is attracted to the transporting belt before it reaches the pressing roller. This may cause a deviation in attraction-force generation timing between the correction rollers and the pressing roller and cause skew to occur again at the leading edge of the sheet material.
In general, some pieces of sheet material are warped in a direction intersecting the transporting direction. When sheet material is warped upward in the middle, if an attraction force from a transporting belt is exerted on the sheet material before the sheet material reaches the nip of the pressing roller, both sides of the sheet material are first attracted to the transporting belt. As a result, the sheet material is pressed against the transporting belt by the pressing roller with the middle of the sheet material floating, and wrinkles occur in the middle of the sheet material.